My Raleigh international (1969?) fits 38's. I had cantilever mounts brazed on. Thinking about fitting wider tires on a modern road frame but the brakes and stays are keeping me off...
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user9042Jan 15 '14 at 21:54

3 Answers
3

With a bike designed around modern "short-reach" 39-49mm calipers, you're going to four main issues:

1) Brake clearance vertically. If everything else is optimal, a 49mm reach caliper should be able to clear around a 32mm tire. You can verify this with an unmounted brake.

2) Brake clearance horizontally. Extra cable tension releases (e.g. in levers or inline) help with this, but the maximum width that the calipers can spread will not be enough to allow an inflated tire through, and even with a deflated tire you might have to force it.

3) Frame clearance. You'll need to have adequate clearance in three places: seatstay bridge, chainstay bridge, and fork crown. In extremely aerodynamic frames, you also may have issues with seat tube clearance.

In general 23mm should be expected, 25mm is likely, and 28mm is occasionally possible. What actually works on a given short reach frame is going to be heavily dependent on the factors above. Bikes with medium-reach (57mm) or long-reach brakes are more capable; I do road riding on a classic 27"-wheeled Trek refitted for 700c with long-reach brakes, 33.3mm tires and full fenders.

This depends entirely on your frame and your brakes. On a racing frame, you likely won't get over a 25, and some won't fit anything larger than 23s. On a commuter frame with fenders, perhaps 28. Without fenders on something like a Surly, you can easily handle 32s. Assuming your brake calipers can reach around the increased tire width.